And those leftover pills pose a risk of misuse, addiction and overdose, researchers at the University of Michigan said.

"It's striking to see the major discrepancy between prescribed amount and the amount patients actually take," said study senior author Dr. Joceline Vu. She is a surgical resident and research fellow at Michigan Medicine.

"This is not a phenomenon of a few outlier surgeons -- it was seen across the state, and across many operations," Vu noted in a university news release.

In the study, the researchers analyzed data from nearly 2,400 patients who had one of 12 common types of surgery at 33 hospitals in Michigan. On average, patients took only 27 percent of the opioids (such as OxyContin) prescribed to them. However, for every 10 additional pills prescribed, patients took five of them.

The median number of pills prescribed was 30 and the median number used was nine, the researchers found.

Patients who had a hernia repair operation -- either open or minimally invasive surgery -- took the most opioids, while those who had their appendix or thyroid taken out took the least, the findings showed.

But the size of the opioid prescription was a more important factor in how many pills a patient took than their pain scores, the intensity of their operation and personal factors, the study authors said.

According to study first author Dr. Ryan Howard, "In what we tell patients about what kind of pain to expect after surgery, and how many pills we give, we set their expectations -- and what the patient expects plays a huge role in their post-operative pain experience. So if they get 60 pain pills, they think they have to take many of them."

Howard, a surgical resident with Michigan Medicine, added, "We hope that by shining a spotlight on the difference between prescription size and actual use, we can empower surgeons to change their prescribing habits, and be a better steward to both their patient and the broader community."